PRIOR ART
The electrolyte used in the conventional electric double layer capacitor is a liquid electrolyte, that is a solution of an electrolyte salt in a polar medium. The electric double layer capacitor has gained in size in recent years as epitomized by the one used as the auxiliary power source of an electric vehicle and is required to have safety and reliability, among others.
However, an electric double layer capacitor utilizing a liquid electrolyte is liable to develop electrolyte leaks so that its safety and reliability are not necessarily satisfactory. This capacitor further has the disadvantage of a high rate of self discharge.
To overcome those disadvantages, much research has been undertaken into polymer type solid electrolytes in recent years. As such polymer solid electrolytes, a complex between poly(ethylene oxide) and an electrolyte salt and a complex between an ethylene oxide-propylene oxide random copolymer and an electrolyte salt (JP-A-62249361), among others, are known.
However, despite improvements in the incidence of electrolyte leakage and self discharge, such polymer solid electrolytes are seriously deficient in ionic conductivity and ion diffusion rate at low temperatures below room temperature so that when any of them is used in an electric double layer capacitor, only a very low capacitance is obtained at such a low temperature and a high discharge current cannot be obtained even In the ambient temperature.